1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in image forming processes such as electrophotography, electrostatic recording or electrostatic printing.
2. Related Background Art
It is conventionally known to form an image on the surface of a photoconductive material by an electrostatic means and then develop the electrostatic image thus formed, for which a large number of methods have been conventionally known, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and various means, and subsequently a toner is adhered to the latent image to form a toner image corresponding to the electrostatic latent image.
Next, the toner image can be transferred to an image holding medium such as paper, followed by the fixation with heat, pressure, heat-and-pressure, or solvent vapor to obtain a copy. When the process comprises a toner-image transfer step, usually the process also comprises the step of removing the remained toner on the photosensitive member.
As developing methods in which the electrostatic latent image is visualized with a toner, there are many known methods such as the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and a method using a conductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258.
As the toners for these developing methods, a fine powder obtained by dispersing a colorant in a thermoplastic resin followed by pulveration are commonly used. As the thermoplastic resin, polystyrene resins are widely used. In addition polyester resins, epoxy resins, acrylic resins, urethane resins, etc. are also used. As the colorant, carbon black is most widely used. In the magnetic toners, black magnetic powders of an iron oxide type are widely used. In a system using a two-component type developer, the toner is usually mixed with carrier particles such as glass beads, iron powder or ferrite particles.
The toner image finally formed on an image bearing medium such as paper or transparent film for overhead projectors (hereinafter "OHP film") is permanently fixed onto the image bearing medium by the action of heat, pressure or heat-and-pressure, of which heat fixation has been widely used.
In recent years, a rapid development is in progress from monochromatic copying to full-color copying, and two-color copying machines and full-color copying machines have been studied for practical use. For example, in Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and ibd. Vol. 25, No. 1, p. 52 (1983) reports relating to color reproduction and gradation reproduction can be found.
The formation of color images in full-color electrophotography uses three-color toners, yellow, magenta and cyan, corresponding to the three primary colors for all color reproduction.
In a method used therefor, first, the light reflecting from an original is passed through a filter which can transmit only specific color ray complementary to the toner color, and an electrostatic latent image is formed on a photoconductive layer. Next, a toner image is transferred on an image bearing medium (or image-receiving medium) through development and transfer steps. This procedure is successively repeated plural times so that the subsequent toner images are superposed on one another on the same support with registration, and thus a final full-color image is obtained through a one-step fixation.
In general, when a so-called two-component type developer comprising a toner and a carrier is used in a developing system, the toner is statically charged by the friction between toner particles and the carrier to the desired level of triboelectricity and charge polarity, and then the electrostatic image is developed with the toner utilizing the electrostatic attraction force. Hence, in order to obtain a good visible image, it is necessary for the toner to have a good triboelectric chargeability which is mainly determined by the relation with the carrier.
Nowadays, to cope with the problems as stated above, research has been done on the materials that constitute developers, for example, research on new materials for carrier core agents and carrier coat agents, as well as the optical amount of them, studies on charge control agents and fluidity-providing agents that are added to toners, and also improvements in binders that serve as mother materials.
For example, Japanese Patent Publication No. 52-32256 discloses a technique in which a charging aid such as chargeable fine particles is added to a toner. Japanese Laid-Open Patent Application No. 56-64352 discloses use of a fine resin powder having a polarity oposite to the toner. Japanese Laid-Open Patent Application No. 61-160760 discloses a technique in which a fluorine-containing compound is added to a developer so that stable triboelectric chargeability can be achieved.
Various proposals are also made on how to add the aforesaid charging aids. For example, it is a common method to make the charging aids attach on the surfaces of toner particles by the action of electrostatic force between toner particles and the charging aid or by van der Waals force, etc. In that instance, a stirrer, a mixer or the like is used. In this method, however, it is not easy to uniformly disperse additive particles onto toner particle surfaces, and free additive particles not adhered to the toner particle surfaces tend to agglomerate on one another, so that it is difficult to eliminate the presence of free additive particles. This tendency becomes more pronounced as the electrical resistivity of the charging aid increases and the particle diameter decreases. In such instances, the performance of toner is affected. For example, the quantity of triboelectricity is liable to be unstable causing non-uniform image density and fogged images, and when copies are continuously taken, the content of the charging aid changes making it impossible to maintain the initial-stage image quality.
As an alternative addition method, the charging aid is previously added together with a binder resin and a colorant when toners are prepared. However, it is difficult to uniformly disperse the charge control agent and it is the charging aid or charge control agent present near the surface of toner particle that can contribute to the chargeability and those present inside of the particles do not contribute to the chargeability. Hence it is not easy to control the amount of the charging aid to be added or the dispersed quantity on particle surfaces. Even in toners prepared by such a method, the quantity of triboelectricity is unsteady, and it is not easy to obtain toners having satisfactory developing performance as stated above.
It has been proposed in Japanese Patent Applications Laid-open No. 59-52255 and No. 60-136755 to use a toner containing hydrophobic titanium oxide. In Japanese Laid-Open Patent Application No. 59-52255, a proposal is made on a toner containing titanium oxide treated with a gaseous alkyltrialkoxysilane. Although the addition of titanium oxide certainly brings about an improvement in electrophotographic performances, further improvement has been sought, because titanium oxide inherently has a low surface activity, and coalescence of particles may occur at the stage of treatment or the powder can not be made uniformly hydrophobic.
Moreover, in recent years, there is an increasing demand for making copying machines have a higher precision and higher image quality. In the technical field, it is attempted to acheive high image quality in color copying by making toner particle diameters smaller.
When the particle diameter of toner particles becomes smaller, the surface area per unit weight increases, tending to bring about an excessive triboelectricity of the toner. This is accompanied with a possibility of the insufficient image density or the deterioration after running. In addition, because of the large triboelectricity of the toner, toner particles have strong mutual attraction force which may result in a low fluidity causing problems in steady toner feeding or in the impartment of triboelectricity to the fed toner.
The color toner such as the yellow toner, the magenta toner or the cyan toner contains no conductive material such as a magnetic material or carbon black. Hence the toner has no part from which triboelectric charge leaks, it tends to have an excessively large quantity of triboelectricity. This tendency is particularly pronounced when a polyester type binder having a high triboelectric chargeability is used as a binder resin of the toner.
In color toners for full-co/or image reproduction, following properties are strongly desired.
(1) On fixing, toner particles must be in an almost completely molten state, to such an extent that their shapes are no longer recognisable so that the fixed toner may not interfere with color reproduction not irregularly reflecting light.
(2) Color toners must have a transparency so that an upper toner layer may not interfere with the color tone of the lower toner layer having a different tone.
(3) Color toner must have well balanced hue and spectral reflection characteristics and a satisfactory chroma.
Nowadays, polyester type resins are widely used as binder resins for color toners. In general toners containing the polyester type resins tend to be affected by temperature and humidity and tend to cause problems such as an excessively large quantity of triboelectricity in an environment of low humidity or an excessively small quantity of triboelectricity in an environment of high humidity. Thus, it is intently sought to provide a color toner and a developer which have a stable quantity of triboelectricity in a wide range of environment.